A gm-ratioed amplifier generally refers to an amplifier whose gain is determined by the ratio of the input transconductance (gm) and load transconductance (gm), wherein the gain is a measure of the ability of the amplifier to increase the power or amplitude of an input signal. The term “gm-ratioed” in this context means transconductance ratioed. That is, at a certain ratio of the input transconductance and load transconductance, the gm-ratioed amplifier may have a fixed gain for small input signals, and by varying the ratio of the input transconductance and load transconductance of the gm-ratioed amplifier, the gain of the gm-ratioed amplifier may be varied accordingly. For example, the ratio of the input transconductance and the load transconductance may be controlled by varying the currents through the transconductance stages (e.g. input stage and load stage) of the gm-ratioed amplifier.
In many applications, linearity is required to be preserved during the amplification of a signal such that a minimum amount of distortion is introduced. In this context, the linearity may refer to the linearity of response of an output signal to an input signal. For example, the input signal may be an input voltage and the output signal may be an output voltage, as is the case for a voltage amplifier. As another example, the input signal may be an input current and the output signal may be an output current, as is the case for a current amplifier. In a further example, the input signal may be an input voltage and the output signal may be an output current, as is the case for a transconductance amplifier. In a still further example, the input signal may be an input current and the output signal may be an output voltage, as is the case for a transimpedance amplifier. In other words, linearity is the behavior that an output signal strength varies in direct proportion to the input signal strength. For example, an amplifier is considered to have a behavior of linearity when the output voltage varies in direct proportion to the input voltage. That is, the output-to-input signal strength ratio remain the same when the amplifier exhibits linearity. Accordingly, when an amplifier exhibits linearity, the gain of the amplifier in decibels (dB) remains to be a constant. The linearity preserves signal shape such that signal correlation properties are protected or information of the edges remains undistorted.
In many designs, the linearity of a gm-ratioed amplifier is achieved at a price of power consumption.